1. Field of the Invention
The present invention relates to a method of preparing an oxide superconducting bulk body, and more particularly, it relates to a method of preparing an oxide superconducting bulk body applied to a strong magnetic field magnet, a superconducting flywheel for storing energy, a superconducting carrying device, a superconducting bearing or the like, which is stably floated for use through the pinning characteristic of the oxide superconducting bulk body under a magnetic field.
2. Description of the Prior Art
Among oxide superconductors, an RE123 superconductor such as a Y123 (YBa.sub.2 Cu.sub.3 O.sub.7-z) or Nd123 (NdBa.sub.2 Cu.sub.3 O.sub.7-z) superconductor has an excellent pinning characteristic and becomes a superconductor having the performance of a magnet in a state capturing magnetic flux when cooled under a magnetic field. Further, such an oxide superconductor enters a normal conducting state from a superconducting state when its temperature is increased beyond the critical temperature. Thus, the oxide superconductor can be readily converted from a magnetic state to a non-magnetic or reverse state at need, to provide an industrial advantage.
As to the chemical formulas of RE123 superconductors having oxygen contents varying with preparation conditions, the atomic ratio of oxygen is expressed as "7-Z" throughout the specification. The value Z is within the range of 0.ltoreq.Z.ltoreq.0.5 in general.
In general, a method of preparing an oxide superconducting bulk body of such a Y123 or Nd123 superconductor is carried out by heating a previously press-molded precursor in an oxide superconducting bulk preparation apparatus and holding the same at a temperature converting the RE123 (REBa.sub.2 Cu.sub.3 O.sub.7-z) superconductor to a liquid phase while not converting an RE211 (RE.sub.2 BaCu.sub.3 O.sub.5) or RE422 (RE.sub.4 Ba.sub.2 Cu.sub.3 O.sub.10) superconductor to a liquid phase, i.e., keeping the same in a solid phase as the maximum temperature. This state, including liquid and solid phases, is referred to as a semi-melted or partially melted state.
For example, Japanese Patent Laying-Open No. 8-133726 (1996) discloses a method of preparing an oxide superconducting material comprising steps of heating a precursor to a temperature range of 950 to 1350.degree. C. as a heat treatment for bringing the precursor into a semi-melted state and thereafter cooling the semi-melted precursor for crystallizing the same.
On the other hand, another literature (Physica C250, 1995, pp. 222 to 226) discloses a method of holding a press-molded pellet of 20 mm in diameter and 10 mm in thickness at 1070.degree. C. for 0.5 hours thereby leaving no solid-phase RE123 semiconductor in a later step.
In practice, an oxide superconducting bulk body must be large-sized to some extent. It has been clarified through electromagnetic calculation that a combination of a plurality of small bulk bodies cannot attain the performance of a large-sized bulk body.
In practical use, therefore, it is indispensable to prepare an oxide superconducting bulk body having a certain measure of size. If the thickness of the bulk body is too large, however, the captured magnetic field is saturated. In order to increase the captured magnetic field, therefore, it is necessary to prepare a bulk body having a large plane size.
If such a large-sized oxide superconducting bulk body is prepared under conditions similar to those generally employed for preparing a relatively small-sized bulk body, however, desired characteristics cannot be attained.